On commercial-scale farms, it is often necessary to dispense doses of medication or nutrients, such as vitamins, to all of the animals in a large herd. To service the animals individually is time consuming and uneconomical. One way of simplifying this procedure has been to add doses of medicine or vitamins to the animal's drinking water. But this raises a new problem, in that some animals would get more than the desired dosage, which might not only be wasteful but actually harmful, whereas other animals would receive too small a dose or none at all. Thus, the problem is one of determining at the dispensing station which of the animal's have received their necessary dosage, and withholding doses from them for a preselected period.